PROJECT SUMMARY Proteasome inhibitor (PI)-based combination chemotherapy is currently being evaluated for the treatment of pancreatic cancer and other solid malignancies. Our hypothesis is that PIs cause endoplasmic reticular (ER) stress in cancer cells and this stress mediates cell killing. Furthermore, we have obtained preliminary evidence that the effects of PIs on cell death are highly heterogeneous and are linked to whether or not they induce phosphorylation of eIF2, a component of the unfolded protein response (UPR) that mediates suppression of global protein synthesis. Specifically, PIs promote strong phosphorylation of eIF2 phosphorylation in the cell lines that are relatively resistant to PI-induced apoptosis, but they fail to do so (or attenuate translation) in the cell lines that are most sensitive. Identifying the biochemical basis for this heterogeneity could enable the prospective identification of tumors that are most likely to respond to PI-based combination chemotherapy and should yield new targets for therapeutic intervention. We also wish to better define the molecular mechanisms involved in the apoptosis that is induced by one of the most promising PI-based combination regimens, namely, the combination of bortezomib plus histone deacetylase (HDAC) inhibitors. We have obtained good preliminary evidence that HDAC inhibitors promote proteasome inhibitor-mediated apoptosis by disrupting cytoprotective structures known as aggresomes that appear to function to alleviate ER stress. Gene silencing studies have demonstrated that the HDAC responsible for aggresome disruption is HDAC6, and it is possible that more selective HDAC6 inhibitors will yield comparable or better tumor cell killing than pan HDAC inhibitors (like SAHA) with less toxicity. To directly test our hypotheses we propose the following Specific Aims. (1) Define the molecular mechanisms that control bortezomib-induced phosphorylation of eIF2. We will test the hypothesis that PIs inhibit PERK activation by inducing the expression of a molecular chaparone (HSP70?) that blocks PERK homoaggregation in drug-sensitive cells; (2) Determine role of ER stress in PI-induced apoptosis. Here we will assess the contributions of ROS, Ca2+, JNK, Noxa, and caspase-4 ot PI-induced apoptosis; (3): Determine the toxicity and anti-tumor efficacy of combination therapy with PIs and HDAC inhibitors in xenografts. We will compare the effects of combination therapy with PIs plus SAHA (a pan HDAC inhibitor), tubacin (HDAC6-selective), or SNDX-275 (type I HDAC-specific) in vitro and in orthotopic tumors derived from sensitive and resistant cell lines. We will also investigate whether or not pharmacodynamic markers of drug-target interaction and biological response can be measured in the peripheral blood of these animals and apply these methods to measure the effects of therapy with bortezomib plus SAHA within the context of a Phase II clinical trial in patients with pancreatic cancer.